Since the invention of nasal Continuous Positive Airway Pressure (nasal CPAP) for treatment of Obstructive Sleep Apnea (OSA) and other forms of Sleep Disordered Breathing (SDB) by Sullivan, as taught in U.S. Pat. No. 4,944,310, much effort has been directed towards improving the comfort of the devices. One aspect of this is a more comfortable patient interface, such as provided by the MIRAGE® and ULTRA MIRAGE® masks manufactured by ResMed Limited. Another aspect of providing a more comfortable patient interface is the comfort of the waveform of air at positive pressure provided by the blower.
Some low cost CPAP blower devices, such as the S7™ device by ResMed Limited, provide a supply of air at a generally fixed positive pressure throughout the respiratory cycle of the patient, for example, 15 cmH2O. A blower comprising an electric motor and fan can be constructed to deliver air based on a rotational speed of the motor predetermined to deliver a particular pressure to a patient interface, such as a mask. When the patient breathes in with such a system, the pressure in the mask may reduce by a small amount. When the patient breathes out with such a system, the pressure in the mask may increase by a small amount. These fluctuations in mask pressure are referred to as “swing”. Other blowers use feedback in a pressure controller which counterbalances the effect of patient effort on the mask pressure to reduces the swing. Such a device has a current retail price in Australia of approximately AU $1000.
Another group of CPAP devices, such as the ResMed AUTOSET® SPIRIT™ device can monitor the patient and determine an appropriate CPAP setting to deliver to the patient, which pressure may vary through the night, for example, delivering 15 cmH2O during an initial portion of the patient's sleep, but increasing to 20 cmH2O later in the night. Changes in pressure are made in response to a determination of the occurrence and severity of aspects of breathing such as flow limitation and snoring. Such a device has a current retail price in Australia of approximately AU $2000.
A bi-level CPAP device, such as the ResMed VPAP® product, provides a higher pressure to the patient's mask during the inspiratory portion of the respiratory cycle, for example, 10-20 cmH2O, and a lower pressure during the expiratory portion of the patient's breathing cycle, for example, 4-10 cmH2O. A mismatch between the device control cycle and the patient respiratory cycle can lead to patient discomfort. When the device makes a transition from the higher pressure to the lower pressure the motor is braked. When the device makes the transition from the lower pressure to the higher pressure, the motor is accelerated. Depending on device settings, the device may be required to make a change of 5-18 cmH2O pressure in 50-100 ms. To achieve this change, the peak power load may be in the order of 60-90 W. Because of the low inertia and peak load requirements of the VPAP® device, a large and expensive power supply is required. Such a device has a current retail price in Australia of approximately AU $3,500-7,500 depending on the device feature set.
U.S. Pat. No. 6,345,619 (Finn) describes a CPAP device that provides air at a pressure intermediate the IPAP (Inspiratory Positive Airway Pressure) and EPAP (Expiratory Positive airway Pressure) pressures during the transition between the inspiratory and expiratory portions of the device control cycle. U.S. Pat. No. 6,484,719 (Berthon-Jones) and U.S. Pat. No. 6,532,957 (Berthon-Jones) describe devices which provide pressure support in accordance with a waveform template. U.S. Pat. No. 6,553,992 (Berthon-Jones et al.) describes a ventilator whose servo-controller adjusts the degree of support by adjusting the profile of the pressure waveform as well as the pressure modulation amplitude. As the servo-controller increases the degree of support by increasing the pressure modulation amplitude, it also generates a progressively more square, and therefore efficient, pressure waveform; when the servo-controller decreases the degree of support by decreasing the pressure modulation amplitude, it also generates a progressively more smooth and therefore comfortable pressure waveform. The contents of all of these patents are hereby incorporated by reference.
CPAP and VPAP devices are mechanical ventilators. Ventilators have been classified (Chatburn, Principles and Practice of Mechanical Ventilation, Edited by M J Tobin, McGraw Hill, 1994, Ch. 2) as being either pressure, volume or flow controllers. In each case, the ventilator controls the pressure of air versus time, volume of air versus time, or flow of air versus time that is delivered to the patient. Many such devices can be programmed to deliver a variety of waveforms, such as pulse (rectangular), exponential, ramp and sinusoidal. The shape of the waveform actually delivered to the patient may be affected by the compliance and resistance of the patient's respiratory system and his breathing effort, as well as mechanical constraints such as blower momentum and propagation delays.
The Siemens Servo Ventilator 900B is a pneumatically powered ventilator which uses a scissor-like valve to control the inspiratory flow pattern (McPherson & Spearman, Respiratory Therapy Equipment, The C.V. Mosby Company, 1985, pp. 469-474).
Ventilators have been constructed to deliver an inspiratory waveform when one of pressure, volume, flow or time reaches a preset value. The variable of interest is considered an initiating or trigger variable. Time and pressure triggers are common. The Puritan Bennett 7200a ventilator is flow triggered. The Dräger Babylog ventilator is volume triggered. The Infrasonics Star Sync module allows triggering of the Infant Star ventilator by chest wall movement. The ventilator's inspiration cycle ends because some variable has reached a preset value. The variable that is measured and used to terminate inspiration is called the cycle variable. Time and volume cycled ventilators are known.
Many ventilators provide a Positive End-Expiratory Pressure (PEEP). Some of these ventilators use a valve (the PEEP valve) which allows the PEEP to be varied. Some devices, such as that taught by Ernst et al. in U.S. Pat. No. 3,961,627, provide a combination of pressure and flow control within one respiration cycle. A control cycle is divided into four phases I, II, III and IV. The respiration cycle and the control cycle do not necessarily have to fall together in time; mostly, however, phases I and II of the control cycle correspond to inspiration, and phases III and IV of the control cycle correspond to expiration. Phases I, III and IV are pressure-regulated, and phase II is flow-regulated. The doctor can choose the pressure course with the three control elements for the expiratory pressure decrease, the inflexion, and the final expiratory pressure. In phase III, the pressure proceeds from the pressure measured at the end of phase II according to a fixed pressure decrease dP/dt. When the pressure measured in phase III reaches the inflexion, the pressure proceeds linearly to the fixed final expiratory pressure. The part of the expiration from the inflexion to the end of the respiration cycle represents phase IV. The linear course of the pressure in the expiration represents a preferred embodiment, but could be replaced by another course of the pressure curve, for example, an exponential.
A spontaneously breathing patient exerts at least some effort to breath, however inadequate. A lack of synchrony between the respiratory cycle of the patient and that of the ventilator can lead to patient discomfort.
In Proportional Assist Ventilation (PAV), as described by Magdy Younes, the ventilator generates pressure in proportion to patient effort; the more the patient pulls, the higher the pressure generated by the machine. The ventilator simply amplifies patient effort without imposing any ventilatory or pressure targets. It is understood that the objective of PAV is to allow the patient to comfortably attain whatever ventilation and breathing pattern his or her control system sees fit. The PAV system is further discussed in U.S. Pat. Nos. 5,044,362, 5,107,830, 5,540,222 and 5,884,662.
In U.S. Pat. No. 5,044,362, Younes describes a system that operates as follows:
(a) Inspired flow: When the high frequency components of the output of a velocity transducer in a line are filtered out, the remaining signal agrees very well with flow measured independently at the airway. Accordingly, the velocity signal in the line is passed through a low pass filter and the resulting signal is used as a command signal for the ventilator unit to produce pressure in proportion to inspired flow, which is said to provide resistive assist. A gain control permits the selection of the magnitude of the assist. In practice, the flow signal is permanently connected to a summing amplifier and a minimum gain is set to offset the resistance of the tubing. When a greater assist is required to offset the patient's own resistance, the gain is increased.
(b) Inspired volume: The signal related to inspired flow may be integrated to provide a measure of inspired volume. A signal proportional to pressure is subtracted to allow for piston chamber compression. The magnitude of the pressure signal that is subtracted is a function of the gas volume of the system, according to Boyle's law. When the resulting signal is routed to the summing amplifier, the ventilator unit develops pressure in proportion to inspired volume. The magnitude of the assist obtained again may be controlled by a gain device.
(c) Ramp generator: This mode of operation permits the ventilator unit to function independent of patient effort and provides a controlled ventilation. This function can be activated by the operator throwing a switch to bring the function generator into the circuit. Alternatively, provision may be made for the ramp generator to be routed automatically to the summing amplifier in the event of the failure of the patient to breathe spontaneously for a specified period of time.
(d) D.C. output: An adjustable DC output provided by an offset amplifier also is routed to the summing amplifier, to result in the generation of continuous pressure.
U.S. Pat. No. 5,535,738 (Estes et al.) describes a further PAV apparatus.
Life support ventilators such as the Puritan-Bennett 7200a allow for a very precise control of the pressure, volume and flow rates of air delivered to patients. However such life support ventilators are too expensive for home use in treating sleep disordered breathing costing in the order of AU $50,000 to 100,000.